Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; and the web of packaging material is sterilized in the packaging unit, e.g. by applying a chemical sterilizing agent such as a hydrogen peroxide solution, which is subsequently removed, e.g. evaporated by heating, from the surfaces of the packaging material.
The sterilized web of packaging material is maintained in a closed, sterile environment, and is folded into a cylinder and sealed longitudinally to form a tube.
The tube is fed in a vertical direction parallel to its axis, and is filled continuously with the sterilized or sterile-processed food product.
The packaging unit interacts with the tube to heat seal it at equally spaced cross sections and so form pillow packs connected to the tube by transverse sealing bands.
More specifically, the unit comprises two forming assemblies movable along respective guides, and which interact cyclically and successively with the tube to heat seal the packaging material of the tube.
Each forming assembly comprises a slide which moves up and down along the respective guide; and two jaws hinged at the bottom to the slide and movable between a closed configuration, in which they cooperate with the tube to heat seal it, and an open configuration, in which they are detached from the tube.
More specifically, the jaws of each forming assembly are moved between the open and closed configurations by respective cams.
The movements of the forming assemblies are offset by a half-period. That is, one forming assembly moves up, with its jaws in the open configuration, while the other forming assembly moves down, with its jaws in the closed configuration, to prevent the assemblies from clashing.
The jaws of each forming assembly are fitted with respective sealing members, which cooperate with opposite sides of the tube, and comprise, for example, a heating member; and a member made of elastomeric material and which provides the necessary mechanical support to grip the tube to the required pressure.
Each forming assembly also comprises two forming members with respective forming half-shells hinged to the respective jaws.
Each two forming half-shells move cyclically between an open position, in which they are detached from the tube, and a closed position, in which they contact the tube and fold the portion of the tube between two consecutive sealing sections to define and control the volume of the package being formed.
More specifically, the sealing device of a first forming assembly seals the bottom of the package being formed, and the half-shells of the first forming assembly control the volume of the package while the sealing device of the second forming assembly seals the top of the package being formed.
More specifically, the forming half-shells may be spring-loaded by respective springs into the open position, and have respective rollers, which cooperate with respective cams designed to move the half-shells into the closed position by the time the forming assembly reaches a predetermined position as it moves down.
Each forming half-shell has a C-shaped cross section, and comprises, integrally, a main wall; and two parallel lateral flaps projecting towards the axis of the tube of packaging material from respective opposite end edges of the main wall.
In the closed position, the main walls are located on opposite sides of the tube axis, are parallel to each other, and cooperate with respective first portions of the tube.
In the closed position, the flaps of one half-shell cooperate with respective second portions of the tube to completely control the volume of the package being formed, and, on the opposite side to the relative main wall, face corresponding flaps on the other half-shell.
Though performing excellently on the whole, packaging units of the type described still leave room for further improvement.
More specifically, a need is felt within the industry to minimize relative slide between the half-shells and the tube of packaging material as the half-shells move from the open to the closed position, so as to prevent marking and/or scratching or, at worst, damage of the packaging material.
When the packaging unit is used to form packages of a larger nominal volume than the volume of the pourable food product inside, i.e. partly empty finished packages, the tube-contacting surface of the main wall of each half-shell has a number of projections, which cooperate with the tube of packaging material to expel part of the pourable food product from the volume of the tube eventually forming the package.
A need is felt within the industry to reduce the amount of pourable food product in the packages, i.e. increase the empty volume of the packages, while at the same time preventing, as far as possible, marking caused by the projections interacting with the package material.